Electrochromic devices, such as electrochromic windows or automotive electrochromic rearview mirrors, change transmissivity with application of voltage and current. The process relies on electrochemical redox (reduction, or gain of electrons and decrease in oxidation state, and oxidation, or loss of electrons and increase in oxidation state) reactions of a material, and is reversible. Cathodic electrochromic materials color or darken cathodically by a reduction process, i.e., when gaining electrons and bleach when giving up electrons. Anodic electrochromic materials color or darken anodically by an oxidation process, i.e., when giving up electrons and bleach when gaining electrons. Ion transmissive (i.e., ion conductive) materials allow ions to travel from one type of electrochromic material to another. Some electrochromic stacks use a cathodic electrochromic material, an ion transmissive material and an anodic electrochromic material.
Electrochromic materials are often slow to change transmissivity, and may do so unevenly in large devices such as electrochromic windows. Gradual, non-uniform coloring or switching is a common problem associated with large area electrochromic devices. This problem, commonly referred to as the “iris effect,” is typically the result of the voltage drop through the transparent conductive coatings providing electrical contact to one side or both sides of the device. For example, when a voltage is initially applied to the device, the potential is typically the greatest in the vicinity of the edge of the device (where the voltage is applied) and the least at the center of the device; as a result, there may be a significant difference between the transmissivity near the edge of the device and the transmissivity at the center of the device. Over time, however, the difference in cell potential between the center and edge decreases and, as a result, the difference in transmissivity at the center and edge of the device decreases. In such circumstances, the electrochromic medium will typically display non-uniform transmissivity by initially changing the transmissivity of the device in the vicinity of the applied potential, with the transmissivity gradually and progressively changing towards the center of the device as the switching progresses. While the iris effect is most commonly observed in relatively large devices, it also can be present in smaller devices that have correspondingly higher resistivity conducting layers.
Another problem with electrochromic materials is that manufacturing methods for electrochromic glass may not be suitable for other materials, especially flexible materials, as substrates, especially since glass has a relatively high melting point as compared to such materials. In particular, standard transparent electrically conductive materials used in electrochromic devices (e.g., transparent conductive oxides, TCOs, such as indium tin oxide, or fluorine doped tin oxide) require high processing temperatures to achieve a combination of low electrical resistance and high transparency. When these standard materials are employed on flexible substrates with limited processing temperatures, then the resistance is higher and/or the transparency is higher. The higher resistance of the electrically conductive layer exacerbates the iris effect because the resistance between the edge and center of the device is larger.
Therefore, there is a need in the art for a solution which overcomes the drawbacks described above, including manufacturing electrochromic devices with spatially coordinated switching on flexible substrates.